I would try installing Visual Studio 2012 Update 3 first.
The sequences I went through were:
1. install Visual Studio 2012.
2. Install the SDK.
3. The tools wouldn't work...(I forget what the error was)
4. Update Visual Studio 2012 to Update 3.
5. all tools worked great.
When I had to rebuild my Windows 7 box, I did the following:
1. Install Visual Studio 2012
2. Install Visual Studio 2012 Update 3.
3. Install the Windows Phone 8 SDK.
4. Everything "just worked" using this sequence.
In your case, make sure Visual Studio 2012 Update 3 is installed.
Weird...it worked fine for my HTC 8x on Windows 7 x64. I'd check your internet connection and make sure the tool can get out to the servers ok.
make sure your date and time are correct on your system before installation. If it is, try setting the date on your system back to Sept 1, 2013, unplug/disable Internet, and try installing again in offline mode.